Screening machine with slotted screen

ABSTRACT

A machine for screening liquid fiber suspension, having a screen with elongated slots disposed along the flow path of the suspension which changes in direction along its length, is made more efficient and plug-free by arranging the slots in at least two areas of the flow path with the long aces of the slots in one area at an angle to the long axes of the slots of the other area such that the slots of said areas have their long axes at substantially a uniform angle, preferably approximately a right angle, to the direction of flow of suspension over them despite difference in said flow direction in the respective areas.

limited States Patent [191 Nelson 1 Jan. 30, 1973 54] SCREENING MACHINEWITH SLOTTED SCREEN g Primary Examiner.lohn Adee [75] Inventor: GeorgeL. Nelson, Wcstwood, Mass. AlmmeyEdgar Kent [73] Assignee: Bird MachineCompang South 7211- [57] ABSTRACT p0 ass A machine for screening liquidfiber suspension, hav- Flledl y 10,1971 ing a screen with elongatedslots disposed along the App]. No.1 141,639

References Cited UNITED STATES PATENTS 7/1899 Smith ..209/273 1/1968Clarke-Bonner ..209/273 flow path of the suspension which changes indirection along its length, is made more efficient and plug-free byarranging the slots in at least two areas of the flow path with the longaces of the slots in one area at an angle to the long axes of the slotsof the other area such that the slots of said areas have their long axesat substantially a uniform angle, preferably approximately a rightangle, to the direction of flow of suspension over them despitedifference in said flow direction in the respective areas.

5 Claims, 3 Drawing Figures SCREENING MACHINE WITH SLOTTED SCREEN Thisinvention relates to the screening of fluids comprising liquidsuspensions of fibrous material and in particular relates to machinesfor screening such suspensions through a slotted screen to separateshives, dirt and other foreign matter from the acceptable fiber.

For screening many liquid-fiber suspensions, particularly thosecontaining relatively long fibers which should not be rejected, screenswith elongated slots should be better suited than round hole screens.However, prior art screening devices with slotted screen plates havesuffered from serious limitations which have made them less thansatisfactory or even unacceptable in many cases. In fine screeningapplications in which narrow slots are required, excessive screen areahas been needed to provide the requisite screening efficiency andcapacity. In addition, the slots become plugged too frequently.Furthermore, the slotted screens have not performed as well astheoretically they should, especially at the high consistencies whichare becoming more common in pulp and paper manufacture.

Accordingly, it is a principal object of the invention to provideslotted screen apparatus which screens more efficiently than existingsuch devices.

Another object of the invention is to provide such screening apparatuswhich is less subject to plugging and easier to maintain than existingdevices.

A still further object is to provide such screening apparatus which iscapable of screening a wide range of liquid-fiber suspensionconsistencies with nearly equal efficiency.

I have found that in slotted screen machines for screening liquid fibersuspensions it is important in obtaining uniform maximum efficiency,capacity and freedom from plugging that the slots be arranged uniformlyat substantially the same angle to the direction of flow of thesuspension past each slot and that generally the selected uniform angleshould be approximately a right angle to the direction of flow. Inscreens of the type in which this invention is concerned, the slots havebeen cut with their long axes parallel to each other and usuallyparallel to the screen plate axis. Since the flow pattern in this typeof screen changes rather drastically as the flow of the suspensionproceeds over the plate from inlet to outlet, the result has been poorperformance because, I have found, of the lack of uniformity inangularity of slots to the direction of flow over them.

According to the present invention a screening machine with slottedscreen plate is provided in which the angularity of the long axes of theslots to the screen axis is not uniform but is varied in generalconformity with change in the angle of flow of the suspension over thescreen as it proceeds from inlet to outlet. This change in angularity ofthe slots is such as to maintain an approximate uniformity of angularrelation of the slots to the direction of flow of the suspension overthem.

In typical pressure screening machines for pulp and paper making fiber,which have a tubular screen and spiral flow of suspension thereover, theflow of suspension across the screen plate changes in angularity to thescreen axis from a rather steep-angled spiral at the inlet end of theorder of 45 to the screen axis, to nearly a circular flow about thescreen axis adjacent the rejects outlet. In a preferred embodiment ofthe invention applied to such a screening machine the slots of the groupadjacent the inlet end are arranged with their long axes atsubstantially a 45 angle to the screen axis while the slots of the groupadjacent the rejects outlet are arranged with their axes substantiallyparallel to the screen axis. Intermediate these two groups there may beprovided one or more groups of slots with their long axes at an angle tothe screen axis intermediate 45 and 0 conforming to the flow angle ofthe suspension to the screen axis in which the group of slots islocated. Preferably the machine includes rotatable pulsing means forproviding positive and negative pulses to the screen and this pulsingmeans is desirably a drum having portions of its surface displacedradially of the drum with respect to other portions to provide thepulsing.

Other objects, features, and advantages will become apparent from thefollowing description of a preferred embodiment of the invention, takentogether with the attached drawing thereof, in which:

FIG. 1 is a side view, partially in vertical cross-section and partiallybroken away, of a preferred screening apparatus according to the presentinvention;

FIG. 2 is a horizontal cross-sectional view of the screening apparatusof FIG. 1, taken on line 2-2 thereof;

FIG. 3 is a perspective view of a cylindrical screen plate constructedin accordance with the invention.

Referring to the screening apparatus shown in FIG. 1, the apparatus hasa vertically arranged pressure casing 10 having a removable pressuredome 12. An annular inlet gutter 14 is defined in the upper region ofcasing 10 and an inlet conduit 16 is arranged to introduce pulp to bescreened into the inlet gutter 14. A gutter trap 18 communicates withinlet gutter 14 for removing heavy debris thrown to the periphery bycentrifugal force.

The inlet gutter 14 communicates radially over a baffle 20 with thecentral part of the screening apparatus. Circumferential, aperturedtubular screen member 22 is located below baffle 20 and has wall 24spaced inwardly relative to casing 10 in the manner to define an annularaccepts chamber 26 outside of screen member 22. A tangential acceptsconduit 28 arranged to remove fluid under substantial pressure isconnected to accepts chamber 26 and has its initial portion extendingthe full height of screen member 22.

Below screen member 22 is arranged an annular rejects gutter 30 incommunication with the inside of the cylindrical screen member 22. Arejects conduit 32 communicating with rejects gutter 30, is arranged toremove fluid under substantial pressure and at a relatively low rate(e.g., not over 20 percent, usually 10 percent or less, of inlet flow),and is provided with valve 34 which serves to regulate the flowtherethrough.

It will be observed that casing 10 is slightly scrollshaped inhorizontal cross-section (FIG. 2), and screen member 22 is arrangedtherein so that accepts chamber 26 increases in radial width graduallyabut its circumference, all the way along its extent until it dischargesto accepts conduit 28.

In this embodiment the pulsing means is rotary drum 36 which hasirregular surfaced wall 38 and cylindrical shape, and has a length atleast equal to that of screen member 22 and a diameter substantially thesame, though with clearance, and is arranged to rotate about screenmember axis 40 on shaft 42, the screen member and drum defining theopposite sides of the screening compartment. Shaft 42 extends into andbelow bearing pedestal 44 and carries pulley 46 which is driven by abelt 48, or series of belts, driven by an electric motor 50 which shouldusually be capable of turning drum 36 to develop a surface speed of atleast 5,000 f.p.m. when the casing is full of fluid. It will berecognized that other forms of pulsing means, such as hydrofoils andpaddles, are suitable for use with the present invention.

Referring to FIG. 3, screen member 22 has two groups, generallydesignated 52 and 54, of slots formed therein. The slots in group 52 areformed in the upper half of member 22 at about a 45 angle to screenmember axis 40 and may be, for example, 1% inch long, 0.016 inch wideand spaced to define generally rectangular slotted sections 56. Theslots of group 52 are inclined to axis 40 forwardly in the direction offlow about the axis (clockwise in the figures) from their ends nearestthe outlet end of the screening compartment, the angle of suchinclination placing their long axes at approximately a right angle tothe direction of flow of the suspension past them. The slots in group 54are formed in the lower half of member 24 with their long axes similarlyinclined at about a angle from member axis 40, which places their axesabout at a right angle to the direction of flow past them. The slots ingroup 54 may, for example, be 1% inch long, 0.016 inch wide and spacedto define generally rectangular slotted sections 60. Groups 52 and 54are arranged in the manner to align the respective axes of the slots 58and 62 as close as is practical in the direction perpendicular to thedirection of stock flow at the location of the particular slots when thescreen apparatus is operating in order to increase the efficiency of theapparatus.

In operation motor 50 is started and drum 36 rotates. Stock underpressure enters inlet gutter 14 through conduit 16, where it flowstangentially and overflows baffle 20, then passes spirally downwardlybetween drum 36 and screen member 22 where it is pulsed by the rotatingirregular wall 38 of drum 36. The rejects portion of the flow isscreened out by member 22 and continues to pass downwardly into rejectsgutter 30 and through rejects conduit 32. The accepts portion flowsthrough the slots in member 22 into accepts chamber 26 and issuesthrough accepts conduit 28. As is known in the art, the smaller thewidth of the slots, the better is the removal of smaller debris from thestock flow. However, when narrower slots are used, the frequency ofscreen clogging increases substantially for a given speed of drumrotation or the drum speed must be increased considerably to maintainthe same low incidence of clogging.

The direction of stock flow in the screening compartment changes inangle as the stock progresses toward the rejects outlet. The flow isspiral about the axis 40 induced by tangential overflow of thesuspension from gutter 14 into the screening compartment, at an angle tothe axis which progressively increases as the suspension flows towardthe outlet end. This change in angle of flow is caused primarily by thelow capacity of the rejects outlet from the screening compartment ascompared with the capacity of the inlet, the escape of suspensionthrough the screen progressively reducing the axial velocity of flow inthe screening compartment with consequent increase in flow angle thereinto the compartment axis.

The angularity of the flow path at various points along the axis can bemeasured by instruments or determined by observation of flow streamers.However, it is more easily calculated with sufficient accuracy forpurposes of the present invention, using basically a wellknown formulafor fluid flow, in a confining body such as a pipe or the screeningcompartment, Q A V, where Q is the quantity of flow past the selectedpoint, A is the cross-sectional area of the confining body at that pointand V is the average velocity of the flow at that point (see, e.g.,Dodge and Thompson, Fluid Mechanics, 1937, pages 74-75). Q is known,being the inlet flow less loss of suspension through the screen, whichis closely proportional to the distance the suspension has traveledalong the screen axis. A is also known (in the screen illustrated it isa constant). Consequently, the formula transposed as V= Q/A is used todetermine the velocity of flow at the inlet (tangential) and the axialcomponent of velocity at selected points along the axis in the screeningcompartment.

The angle of flow (angle of velocity) at any point is then determined asthe resultant or vector of the axial and tangential components ofvelocity at that point (e.g., dividing the axial component by thetangential component gives the cotangent of the flow angle from theaxis). The tangential component of velocity is increased above the inletvelocity by acceleration of the rotor proportionately as the suspensionflows in contact with the rotor, which increase may total approximatelypercent. Therefore, the tangential component of velocity at the selectedpoint may be taken as the inlet velocity plus the acceleration thereofup to that point.

For example, with a 2,000 g.p.m. inlet flow and 10 percent thereofrejects outlet flow in apparatus of the design shown, the angle ofsuspension flow in the screening compartment was so calculated forvarious points along the screen to yield the following results:

location axial tangential angle of along component component velocity(flow) screen of of velocity from axial plate velocity inlet endvelocity 14.15 ft/sec l0 ft/sec 35 k 10.97 ft/sec l2 ft/sec 48 1% 7.78ft/sec l4 ft/sec 61 34 4.60 ft/sec 16.5 ft/sec 74 outlet end 1.412ft/sec 20 ft/sec 86 1. In a screening machine for liquid-fibersuspension having a tubular screen with elongated slots, said screendefining a wall of an annular screening compartment surrounding an axis,an inlet to said compartment adjacent one end of said screen, an outletadjacent the other end of said screen for suspension rejected by saidscreen and a separate outlet for suspension passing through said screen,said outlet for rejected suspension having a low capacity relative tosaid inlet, said inlet arranged to feed suspension into said compartmentin a flow which spirals about said axis toward said outlet, the angle ofsaid flow to said axis increasing progressively and substantially as theflow proceeds along said axis, the improvement wherein a firstmultiplicity of said screen slots adjacent the inlet end of saidcompartment are arranged substantially parallel to one another and at asubstantial angle to a second multiplicity of said screen slots arrangedsubstantially parallel to one another adjacent its outlet end, and saidfirst slots are inclined to said axis at a substantially greater anglethan said second slots, said first slots having their ends nearest theinlet further in the direction of flow about said axis than their endsnearest the outlet from said compartment.

2. A screening machine according to claim 1 wherein said angle of saidfirst slots to said axis is about 45.

3. A screening machine according to claim 1 wherein said second slotsare substantially parallel to said axis.

4. A screening machine according to claim 1 which includes a pulsingmeans spaced opposite said screen and rotatable about said compartmentaxis.

5. A screening machine according to claim 4 wherein said pulsing meanshas a substantially continuous surface provided with portions displacedfrom other portions to produce pulses.

UNITED STATES PATENT crncn QERTEWCATE @E QQRREQHGN Patent No. 2 .71?Jill-1 Dated January Q0 n 10']? Invent r( Gecr e L on It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

The second reference cited change the patent number from "336,759" to--3,363,759-- and change the inventor from "Clarke-Bonner" to--C1arke-Pounder--. Y

The abstract, line 6, change "aces" to --axes-.-.

Col. A, line. 5 of the table, second column, delete "velocity" before l.i .15 ft/sec".

Col. '4, line 6 of the table, first column, change "1/2" to --l/4-.

Col. 4, last line of'the table, second column, change 1. 412 ft/sec" to--l. ll5 ft/seo. v

Signed and sealed this 26th day of June 1973.

(SEAL) Attest;

EDWARD M.PLETci1ER,JR, f ROBERT GOTTSCHALK Attesting OfficerCommissioner of Patents FORM F'O-1 USCOMM-DC 60376-P69 U.S. GOVERNMENTPRINTING OFFICE 1969 0-366-334

1. In a screening machine for liquid-fiber suspension having a tubularscreen with elongated slots, said screen defining a wall of an annularscreening compartment surrounding an axis, an inlet to said compartmentadjacent one end of said screen, an outlet adjacent the other end ofsaid screen for suspension rejected by said screen and a separate outletfor suspension passing through said screen, said outlet for rejectedsuspension having a low capacity relative to said inlet, said inletarranged to feed suspension into said compartment in a flow whichspirals about said axis toward said outlet, the angle of said flow tosaid axis increasing progressively and substantially as the flowproceeds along said axis, the improvement wherein a first multiplicityof said screen slots adjacent the inlet end of said compartment arearranged substantially parallel to one another and at a substantialangle to a second multiplicity of said screen slots arrangedsubstantially parallel to one another adjacent its outlet end, and saidfirst slots are inclined to said axis at a substantially greater anglethan said second slots, said first slots having their ends nearest theinlet further in the direction of flow about said axis tHan their endsnearest the outlet from said compartment.
 1. In a screening machine forliquid-fiber suspension having a tubular screen with elongated slots,said screen defining a wall of an annular screening compartmentsurrounding an axis, an inlet to said compartment adjacent one end ofsaid screen, an outlet adjacent the other end of said screen forsuspension rejected by said screen and a separate outlet for suspensionpassing through said screen, said outlet for rejected suspension havinga low capacity relative to said inlet, said inlet arranged to feedsuspension into said compartment in a flow which spirals about said axistoward said outlet, the angle of said flow to said axis increasingprogressively and substantially as the flow proceeds along said axis,the improvement wherein a first multiplicity of said screen slotsadjacent the inlet end of said compartment are arranged substantiallyparallel to one another and at a substantial angle to a secondmultiplicity of said screen slots arranged substantially parallel to oneanother adjacent its outlet end, and said first slots are inclined tosaid axis at a substantially greater angle than said second slots, saidfirst slots having their ends nearest the inlet further in the directionof flow about said axis tHan their ends nearest the outlet from saidcompartment.
 2. A screening machine according to claim 1 wherein saidangle of said first slots to said axis is about 45*.
 3. A screeningmachine according to claim 1 wherein said second slots are substantiallyparallel to said axis.
 4. A screening machine according to claim 1 whichincludes a pulsing means spaced opposite said screen and rotatable aboutsaid compartment axis.